"More than 40 percent of the total number of cases have occurred within the past 21 days. However, most cases are concentrated in only a few localities," the United Nations health agency said in a statement.

The WHO is later due to launch a new strategic plan for tackling the spread of the virulent disease.

A separate Ebola outbreak in the Democratic Republic of Congo, identified as a different strain of the virus, is not included in the latest figures which cover Guinea, Liberia, Sierra Leone and Nigeria.

The disease has overwhelmed West Africa's already fragile health infrastructure. On Wednesday, the head of the African Development Bank said it was causing enormous damage to the economies of the region.